Degradation of acetaminophen by Fenton and electro-Fenton processes in aerator reactor

Abstract

This study investigated the degradation of acetaminophen (ACTP) in the Fenton and electro-Fenton processes in an aerator reactor. The influences of important parameters on acetaminophen degradation were studied and described in details. The experimental results show that the Fenton and electro-Fenton processes had maximum ACTP degradation efficiencies of 99% and 100%, respectively. The ACTP degradation efficiency of Fenton process and electro-Fenton process were apparently increased at pH 2 and 4, respectively; ACTP degradation efficiency increased 72% at pH 2 in the Fenton process and 74% at pH 4 in the electro-Fenton process when fixed H₂O₂ dosage (15 mM) and Fe²⁺ dosage was increased from 0.01 to 0.1 mM. The Box–Behnken design results indicated that the relative effects of the studied parameters on ACTP degradation in Fenton and electro-Fenton processes were Fe²⁺ > H₂O₂ > pH. The initial rate of ACTP degradation (r) and ACTP degradation efficiency in the electro-Fenton process were apparently higher than those in the Fenton process at pH 2, as a high H₂O₂-to-Fe²⁺ molar ratio (1500) was used. The COD and TOC obtained by electro-Fenton process were 12% and 65% higher, respectively, than those in the Fenton process.

Introduction

Huge amounts of commercial pharmaceuticals are used worldwide for medical and agricultural purposes. A current environmental issue is the effects of pharmaceuticals and personal care products (PPCPs). Human use of PPCPs results in excretion of unchanged constituents of PPCP through urine and faeces or as metabolites via municipal sewage systems. The PPCPs have been detected in waste streams and effluents from hospitals, wastewater treatment plants and livestock [1], [2], [3] and are considered a major emerging contaminant [4].

The various treatment technologies for removing or degrading PPCPs include advanced oxidation processes (AOPs) [5], which were defined in a 1987 study by Glaze et al. [6]. These processes generate hydroxyl radicals (OH) [7], [8], [9], which are powerful non-selective oxidants. Therefore, they can oxidize and mineralize almost all organic compounds into CO₂ and inorganic ions [10], [11], [12], [13], [14]. A common AOP is the Fenton process, which is initiated by hydroxyl radicals formation during a Fenton reaction (Eq. (1)) [15].

$${\text{Fe}}^{2+}+{\text{H}}_2{\text{O}}_2\to {\text{Fe}}^{3+}+{\text{OH}}^{-}+{}^{}\text{OH}{k}_1=70{\text{M}}^{-1}{\text{S}}^{-1}$$

In an acidic medium, this serial complex reaction generates the radicals shown in the following equations:

$${}^{}\text{OH}+{\text{Fe}}^{2+}\to {\text{Fe}}^{3+}+{\text{OH}}^{-}{k}_2=3.2\times {10}^8{\text{M}}^{-1}{\text{S}}^{-1}$$

$${\text{Fe}}^{3+}+{\text{H}}_2{\text{O}}_2\to {\text{FeOOH}}^{2+}+{\text{H}}^{+}{k}_3=0.001-0.01{\text{M}}^{-1}{\text{S}}^{-1}$$

$${\text{FeOOH}}^{2+}\to {\text{Fe}}^{2+}+{}^{}{\text{O}}_2\text{H}$$

$${\text{Fe}}^{2+}+{}^{}{\text{O}}_2\text{H}\to {\text{Fe}}^{3+}+{\text{HO}}_2^{-}{k}_5=1.3\times {10}^6{\text{M}}^{-1}{\text{S}}^{-1}$$

$${\text{Fe}}^{3+}+{}^{}{\text{O}}_2\text{H}\to {\text{Fe}}^{2+}+{\text{O}}_2+{\text{H}}^{+}{k}_6=1.2\times {10}^6{\text{M}}^{-1}{\text{S}}^{-1}$$

$${}^{}\text{OH}+{\text{H}}_2{\text{O}}_2\to {\text{H}}_2\text{O}+{}^{}{\text{O}}_2\text{H}{k}_7=3.3\times {10}^7{\text{M}}^{-1}{\text{S}}^{-1}$$

$${}^{}\text{OH}+\text{organics}\to {\text{H}}_2\text{O}+\text{production}$$

In the Fenton process, the ferrous ion (Fe²⁺) dosage is added to catalyze hydroxy peroxide (H₂O₂), which generates ferric hydroxide sludges that require an additional separation process and disposal. Application of the electrochemical method in Fenton process, which is known as the electro-Fenton (EF) process, can be performed in three ways [16]. The first approach is to apply ferrous ions so that hydrogen peroxide and ferrous ion are concurrently generated at the cathode. The second approach is to apply hydrogen peroxide and use an iron anode as a ferrous ion source [16] or to electrogenerate ferrous ion by reducing ferric hydroxide sludge [17]. The third approach is to electrogenerate ferrous ion and hydrogen peroxide at a sacrificial anode and cathode, respectively [18]. The studies have shown that toxic and refractory organics including dyes in wastewater can be destroyed by the electro-Fenton application [19], [20].

In the pharmaceuticals and personal care industries, acetaminophen paracetamol (ACTP) is among the most used and abused drugs in the world today. Recent studies have attempted to use AOPs for acetaminophen treatment [21], [22], [23], [24], [25]. These works have shown that electro-Fenton process can regenerated Fe²⁺ at the cathode, which also minimizes sludge production. However, no studies have compared acetaminophen degradation between Fenton and electro-Fenton processes. This study therefore compared acetaminophen degradation, chemical oxygen demand (COD) and total organic carbon (TOC) between the Fenton and electro-Fenton processes under similar batch mode experimental conditions. The Box–Behnken statistical design was applied to determine the optimum conditions for acetaminophen degradation.